This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-267786, filed Sep. 4, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a profile measuring method and apparatus for use in the field of manufacturing of an ultra high degree precision machine components, the field of managing their quality and/or the field of measuring them, the surface profile of a preformed product, such as a magnetic recording component and an LSI substrate, or a liquid crystal substrate for computers, the method and apparatus capable of accurately measuring, by uses of the interference of light, with the ultra high degree precision ranging from several nanometers to several hundreds of micrometers.
2. Description of the Related Art
Techniques for measuring the shapes, in particular, surface profiles of industrial products, are important industrial techniques, and profile measurement apparatuses employing various measuring methods are now being used. In particular, profile measurement apparatuses use of the interference of light capable of non-contact measurements are widely used. However, most apparatuses of this type are not made in JAPAN.
In general, the profile measuring methods by uses of the interference of light, white light is applied to the surface of a sample through a band-pass filter, interference light formed by light reflected from the surface of a sample and light reflected from a reference surface (a reference mirror) is picked up by a camera, and an image output from the camera is subjected to image processing (the image output is analyzed). In the profile measuring methods for acquiring interference light include, for example, the phase shift method or multipoint simultaneous pickup method. The phase shift method include various types of making an optical-path difference for the acquisition of interference light. Further, there are various types of output image processing methods.
Wyco Corporation (USA), for example, provides a profile measurement apparatus in which an interference phase is obtained for a smooth surface having an unevenness of about several microns, using the phase shift method that employs a shift amount of xcfx80/2, while the profile of a surface having a greater unevenness is obtained from the peak position of an envelope indicative of variations in the sine wave of an interference signal appearing when an optical-path difference is changed.
In the Wyco profile measurement apparatus, to enable both a smooth surface and a rough surface to be measured, phase shifting is executed in units of less than xcfx80, such as xcfx80/2 or xcfx80/3. A number of interference images are picked up by a two-dimensional camera, and phase data of each pixel of the camera is obtained from a signal output from the camera, using the phase shift algorithm. Further, the envelope of variations in the sine wave of an interference signal is extracted from the signal, thereby calculating the profile of the surface.
Zygo corporation (USA) provides a measurement apparatus for picking up an interference image using a shift amount of xcfx80/2, and simultaneously extracting interference contrast data and phase data, using a data analysis in frequency domain.
In the Zygo profile measurement apparatus and method, it is not necessary to measure the peak position of an envelope for each pixel, and both a smooth surface and a relatively rough surface with a step can be measured using the same data processing method. Further, this method incorporates a technique for minimizing the required memory capacity.
In addition to the above-described two measuring apparatuses, a further method has been proposed. In this method, the shape of an envelope of variations in the sine wave of an interference signal, which appears when an optical-path difference is changed, is estimated from a small number of sampling points (in terms of an amount of shift (shift amount) in the phase shift method, image pickup is executed in units of about 10xcfx80) using a band-pass type sampling theorem, thereby extracting the peak position and obtaining a surface profile.
Moreover, a method for increasing measurement speed has been proposed, which employs multipoint simultaneous image pickup using a confocal microscope effect and a microlens array (Journal of the Japan Society for Precision Engineering, Vol. 64, No. 7, pp 1022 to 1028, 1998).
In the case of using light interference based on light of a wavelength xcex, if an optical-path difference is differed with xcexxc3x97nxe2x80x2 (nxe2x80x2 is an integer), the intensity of interference light is substantially identical for different values of nxe2x80x2, and hence the optical difference cannot be correctly determined. Accordingly, a sample that has a rough surface with discontinuous surface level variations cannot be measured simply by a method for obtaining a phase value.
In the aforementioned Wyco profile measurement apparatus, phase shifting is executed in units of less than xcfx80, such as xcfx80/2 or xcfx80/3, and a number of light interference images are picked up by a two-dimensional camera. In this case, usually, about {fraction (1/30)} second is required to transfer data corresponding to one image from the camera to a computer, and the speed, at which the optical-path difference of the interferometer is changed, is set to, at maximum, (xcex/4)/({fraction (1/30)} sec.). In other words, if a phase shift amount is just xcfx80/2, the image pickup operation must be executed four times in order to change the optical-path difference by one wavelength. This means that the interferometer is of a reflection type, and the movement speed of an incorporated movable table is about 2.0 xcexcm/sec., and much time is required for measuring a rough surface.
In this measuring method, even if a high-speed camera is used to increase the transfer rate of image data, much time is required for data processing since the amount of image data itself cannot be reduced, thereby making it difficult to considerably increase the measurement speed.
Furthermore, in the Zygo measuring method and measurement apparatus, a phase shift amount employed in its three-dimensional profile measuring method is about xcfx80/2, as in the Wyco apparatus, and hence it is difficult to significantly increase the measurement speed.
On the other hand, in the method in which the shape of an envelope of variations in the sine wave of an interference signal, which appears when an optical-path difference is changed, is estimated from a small number of sampling points (in terms of a shift amount in the phase shift method, image pickup is executed in units of about 10xcfx80), using a band-pass type sampling theorem, thereby extracting the peak position and obtaining a surface profile, the time required for measurement can be shortened (the measurement speed can be increased), but the measurement precision is about {fraction (1/10)} to {fraction (1/100)} of that of the phase shift method.
The other method, which employs multipoint simultaneous image pickup using a confocal microscope effect and a microlens array, merely provides a measurement precision as low as that obtained by a microscope using confocal focusing (confocal microscope).
It is the object of the invention to provide a profile measurement apparatus and method for measuring, at high speed and in a non-contact manner, the surface profile of a to-be-measured object with a precision as high as that of the phase shift method.
According to an aspect of the present invention, there is provided a profile measuring method of calculating, using a phase shift method, a phase value of an interference image formed by a light beam reflected from an object and a light beam reflected from a reference mirror, thereby obtaining an optical path difference from the calculated phase value and obtaining a profile of the object from the optical path difference, comprising: emitting, to the object, two flash light beams, having wavelengths slightly different from each other, with a predetermined interval t1 therebetween; and picking up, using a camera, interference light formed by light beams reflected from the object and a light beam reflected from the reference mirror, while moving the object in a direction, in which the two flash light beams are directed, in units of intervals t2 at which each of the two flash light beams is cyclically emitted, a phase shift amount corresponding to a movement amount of the object at a time being set to a value falling within a range of 2nxcfx80xc2x1xcfx80/2xc2x1xcfx80/4.
According to an other aspect of the present invention, there is provided a profile measurement apparatus comprising: a first flash light source which emits a flash light beam of a first wavelength; a second flash light source which emits a flash light beam of a second wavelength, different from the first wavelength, with an interval t1 from the light beam of the first wavelength; a movement table which moves an object in a direction in which the flash light beams from the first and second flash light sources are directed to the object; a translucent mirror which diverges, to a reference mirror, part of the two flash light beams emitted from the first and second flash light sources and directed to the object, the translucent mirror returning, to an original route, light reflected from the reference mirror; a camera which picks up, as an image, interference light formed by light reflected from the object and light reflected from the reference mirror; a table control section which moves the movement table by a predetermined distance in synchronism with an interval t2 between successive emissions of light from the first and second flash light sources; and an image processing unit which selects an image signal with a maximum amplitude from a plurality of images output from the camera when the movement table has been moved by the predetermined distance in synchronism with the interval t2, thereby determining an optical path difference on the basis of a phase difference between two interference images created by the two flash light beams emitted from the first and second flash light sources when the camera has picked up the image signal with the maximum amplitude, and also on the basis of phase values of the interference images assumed when the camera has picked up the image signal with the maximum amplitude.
According to a still other aspect of the present invention, there is provided a profile measurement apparatus comprising: a first flash light source which emits a flash light beam of a first wavelength; a second flash light source which emits a flash light beam of a second wavelength, different from the first wavelength, with an interval t1 from the light beam of the first wavelength; a movement table which moves an object in a direction in which the flash light beams from the first and second flash light sources are directed to the object; a translucent mirror which diverges, to a reference mirror, part of the two flash light beams emitted from the first and second flash light sources and directed to the object, the translucent mirror returning, to an original optical path, light reflected from the reference mirror; a camera which picks up, as an image, interference light formed by light reflected from the object and light reflected from the reference mirror; a table control section which moves the movement table by a predetermined distance in synchronism with an interval t2 between successive emissions of light from the first and second flash light sources; and an image processing unit which selects an image with a maximum amplitude from a plurality of images output from the camera each time the first and second flash light sources emit their respective light beams with the interval t1 interposed therebetween, and also each time the movement table is moved by the predetermined distance in synchronism with the interval t2, the image processing unit then calculating an optical path difference from two interference images formed by the two light beams emitted from the first and second flash light sources and reflected from a surface of the object, the image processing unit calculating respective optical path differences at the points, thereby determining a surface profile of the object on the basis of phase values of the two flash light beams emitted from the first and second flash light sources when the camera has picked up an image signal with a maximum amplitude from the output images at each of the points of the to-be-measured area of the object.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.